AQA AS Level Physics Paper 2, June 2023: Question 20

1 mark · Medium difficulty · Multiple Choice

Identify the correct graph showing the variation of resistivity with temperature for a superconducting material.

Practise this question

Question

Multiple choice question showing four graphs labelled A, B, C, and D of resistivity rho against temperature T. Graph A shows resistivity at a constant low level before rising linearly. Graph B shows resistivity dropping to zero at Tc then rising linearly. Graph C shows zero resistivity up to Tc and then jumping to a constant high value. Graph D shows zero resistivity up to Tc, then a sharp vertical jump followed by a gradual linear increase with temperature.
Question text

20 A superconducting material has a critical temperature Tc.

Which graph shows the variation of resistivity ρ with temperature T?

[1 mark]

A

B

C

D

Mark scheme

Show the mark scheme The mark scheme indicates that the correct answer is option D, showing the graph with zero resistivity up to Tc, a vertical jump, and a rising resistivity thereafter.

20 D

How to answer it

Superconductivity and Resistivity Graphs

What this question tests

This question assesses your understanding of the electrical properties of superconductors, specifically the definition and behaviour of the critical temperature (Tc) and how a material's electrical resistivity (ρ) drops sharply to zero when cooled below this threshold.

Question 20 (1 Mark)

Variation of Resistivity with Temperature

✅ Correct Answer: Graph D

Graph D is the correct choice. It clearly shows that for temperatures below the critical temperature ( T < Tc ), the resistivity is exactly zero. At T = Tc , there is a sudden, vertical drop to zero resistivity, and above Tc , the resistivity increases linearly with temperature like a normal metal.

💡 Key Knowledge

  • Superconductor: A material that has zero electrical resistivity below a critical temperature.
  • Critical Temperature (Tc): The temperature at which a material's resistivity abruptly drops to zero.
  • Normal Conductor Behaviour: Above Tc , resistivity increases linearly as temperature increases due to increased lattice vibrations scattering electrons.

🧠 Exam Technique

When tackling multiple-choice graph questions in physics, break them down into distinct conditions:

  1. Check the value of ρ at T = 0 . It must be zero for a superconductor.
  2. Look for the abrupt vertical transition at Tc .
  3. Verify the behaviour above Tc (it should behave like a normal conductor with positive gradient).

❌ Common Errors

  • Graph A: Shows zero resistivity from 0 up to Tc , but the transition is a gradual slope rather than a sharp drop.
  • Graph B: Incorrectly shows resistivity decreasing to zero at Tc and then rising again symmetrically, confusing superconductivity with semiconductor or V-shaped phenomena.
  • Graph C: Shows resistivity dropping to zero above Tc and staying constant, which completely inverts the definition of critical temperature.
Mark Scheme Allocation: 1 mark awarded for selecting D. No working out required.

Topics

Physics · 3.5 Electricity

Question and mark scheme from the AQA AS Level Physics examination, Paper 2, June 2023. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.